The Chinese government snubbed a U.S. request for help in cracking down on a string of alleged investment frauds that have cost Americans billions, outgoing Securities and Exchange Commission Chairman Mary Schapiro told ABC News.

The lack of cooperation has stymied efforts to recoup investor losses, she said, in one of the largest sprees of alleged financial crimes in recent memory -- one that has gone largely unnoticed by most Americans.

"The consequence is it's very much more difficult for us to prove our cases," Schapiro said in one of her final interviews before leaving the post, which will be broadcast tonight as part of an ABC News investigation airing on "World News with Diane Sawyer" and "Nightline."

More than 100 China-based companies have now been de-listed, have left the NASDAQ and New York stock exchanges, have been denied listing, or have withdrawn applications, all following allegations of fraud or accounting irregularities, the ABC News investigation found.

Not every Chinese company was implicated -- many continue to thrive. But experts estimate that Americans -- everyone from small investors to hedge-fund titans -- have lost tens of billions of dollars in the suspect Chinese investments. Prosecutors in the Bernie Madoff case calculated that his investors lost about $20 billion in his decades-long Ponzi scheme.

Schapiro said the SEC opened 40 cases against Chinese firms during her tenure, financial schemes she described as "brazen" and "extraordinary." Schapiro, who stepped down in December, said that when she asked Chinese Vice Premier Wang Qishan for help during a trip to Beijing in July her requests were rebuffed.

"We haven't yet achieved a level of cooperation that makes it possible for us to get access to Chinese companies the way we need," Shapiro said. "We will fight hard to try to secure recovery for U.S. investors. But it's harder when we don't have the cooperation of the foreign government."

The wave of cases is unique because the China-based companies were listed on U.S. stock exchanges, where investors were expecting corporate financial information to be transparent, accurate and verified. In case after case, Schapiro said, questions emerged about the numbers coming from China.

NASDAQ CEO Robert Greifeld wrote a letter to ABC News in response to questions about the issue, saying the exchange has taken a series of "decisive" steps aimed at protecting investors and removing "bad actors" from the listings.

"We have taken prompt, appropriate action to protect investors and the market," Greifeld said.

Later, when ABC News approached NASDAQ officials to address questions on camera about how so many allegedly fraudulent companies came to be listed in the first place, reporters were escorted off of NASDAQ property.

Experts tell ABC News the frauds were fueled by a burst of interest among U.S. investors in putting money behind the Chinese industrial boom.

Companies were able to exaggerate and in some cases fabricate their earnings reports, hiding behind an opaque Chinese financial system that made it difficult for auditing firms to verify the numbers. Many of the companies were located in rural parts of China where it could be difficult to verify that factories were actually producing the goods they had been describing to investors on glossy brochures and in presentations at lavish conferences.

"In 2009, everybody wanted to be in on it," said Dan David, vice president of GeoInvesting, LLC, a firm that monitored the Asian investment craze. "Then we started to see reports from people on these companies that said that there were accounting errors and material misrepresentations. And then it started to grow from there that there was outright fraud."

David told ABC News that collectively, the epidemic of China-based frauds has grown into one of the most costly crime sprees in history.

Among the most notable cases was one involving Puda Coal, a rural Chinese coal company that is alleged to have sold off its coal mine without ever telling investors.

Another company accused of fraud is China Integrated Energy, a large Chinese energy firm, one of whose biodiesel plants was supposed to be producing fuel at maximum capacity of 100,000 tons a year. Jon Carnes, a so-called short seller, whose investment strategy involves profiting when the value of a company goes down, began to question the company's production reports in 2010 and decided to send investigators with a camera to see what really was occurring where the plant was located, in the remote city of Tongchuan. The resulting time-lapse video appears to show an almost dormant plant, where only six tanker trucks stopped to fill up over the course of four months.

"During that period, I found that they produced, essentially, nothing," Carnes said.

Carnes's report sparked a frenzied response in the U.S., as investors began to question the profit statements from the company.

The company responded by conducting an internal investigation. "While some issues remain as to production at the Company's Tongchuan biodiesel facility, and while the investigation revealed the need to strengthen internal controls and take similar measures, the primary substance of all other allegations has been proven groundless," the company's report said. In the aftermath of Carnes's report, the company's auditor resigned and NASDAQ moved to remove its listing from the exchange.

As fraud cases involving China-based firms began to mount over the past three years, the SEC began issuing warning letters to investors about the risks involved. And the two major U.S. stock exchanges began taking steps to address what had clearly emerged as a problem.

For several years, NASDAQ, NYSE and the exchange it owns that was known as the AMEX had all been allowing Chinese companies to be listed through a process called a "reverse merger" that was quicker than the more traditional approach of an initial public offering. The firms would merge with dormant companies that had already attained an SEC registration, speeding up the path to getting listed on a stock exchange.

Officials with the New York Stock Exchange noted that the vetting of Chinese firms was the responsibility of multiple parties. "It's easy to focus in on the exchanges, but there's a food chain involved here," an NYSE official told ABC News. "When a company lists, it involves law firms and bankers and regulators. Then you have the accounting firms, who are really critical to this process. When we list a company you need those audited and verified financials. That's what we all depend on."

Both exchanges have since adopted a seasoning period for new listings, forcing applicants to provide a full year of audited financials before they could trade on the market.

In response to written questions from ABC News, NASDAQ CEO Greifeld said the seasoning period was one of "a number of decisive and fair steps" taken in 2010, once the markets began seeing evidence of a problem.

The NASDAQ imposed more stringent qualifying criteria for companies using "reverse mergers" to get listed, Greifeld said. It required listing companies to reconcile their SEC reports with tax filings, and verify their cash balances. It hired an outside investigative firm to look into the background of applicants for listings and had an internal team investigate allegations of financial misstatements or fraud.

"In the last two years, the efforts of our team has directly led to the delisting of over 40 Chinese companies where there was evidence of fraud, financial misstatement, or other failures to satisfy our listing rules," Greifeld said in his letter to ABC News. "The results speak for themselves."

By the time some of the precautionary steps took effect at the U.S. exchanges, a great deal of money had already been lost.

Even some well-known names in the investment world were reported to be victims. The hedge fund run by John Paulson -- who is best known for predicting the collapse of the housing market -- was reported to have lost $468 million on an investment in Sino Forest, a company listed on a Canadian exchange that was accused of exaggerating its timber holdings. Starr International Co., a firm run by former American International Group CEO Maurice "Hank" Greenberg, sued China MediaExpress Holdings Inc. after he said he was deceived by the company's misleading financial statements. He had invested about $13.5 million in the company. Both companies disputed the allegations against them, but lost their stock exchange listings after fraud allegations surfaced.

Most victims were not Wall Street movers and shakers but small investors who held stock in China-based companies through personal investment accounts, mutual funds or pension funds. Al Smith, 65, the manager of a truck stop in Boise Idaho, told ABC News he lost much of his retirement savings -- about $60,000 -- when he took a chance on Puda Coal, a stock he said he found while researching investment information about Chinese companies.

He was on vacation when allegations surfaced that the coal company's CEO had sold off the coal mine without telling investors. By the time Smith got back to his computer to sell off his shares, trading in the stock had halted. His money was gone.

ABC News went to Puda Coal headquarters to attempt to speak with the company's chairman, Ming Zhao, but his assistant told the reporters that he had not been in the office in weeks, and they did not know when he would return. The SEC has also been pursuing Zhao, but has never received a reply, Schapiro said. She said Americans have little legal recourse -- Americans cannot sue companies in Chinese courts, and people like Zhao remain beyond the reach of the American justice system.

Smith's lawyer, Rob Prongay, who is overseeing a lawsuit against Puda Coal, said he believes a number of entities failed his client.

"Ultimately, there are a variety of people responsible for the harm investors have suffered as a result of investing in what turned out to be fraudulent Chinese companies," Prongay said. "In our view, the auditors, underwriters, and directors, are also liable for what occurred and we are attempting to maximize any potential recovery for the damaged investors."

Calls to the Chinese Embassy in Washington and emails sent to Ming Zhao's attorney in Beijing were not returned.

Schapiro said she too heard little in the way of a response from the Chinese government.

"We're working hard with the Chinese government to try to secure their cooperation for our enforcement investigations," she said. "But we're not going to wait for their cooperation. We are going to proceed and have proceeded to bring cases against more than 40 individuals and entities for fraud on U.S. investors from Chinese companies."

ABC News producer Karson Yiu contributed to this report from China. Stuart Johnson contributed to the reporting on this story as an intern in the ABC News investigative unit in Washington, D.C.